Raman fiber amplifier communication systems

ABSTRACT

The specification describes an improved optical fiber design in which the criteria for high performance in a Raman amplified optical system, such as moderate effective area, moderate dispersion, low dispersion slope, and selected zero dispersion wavelength, are simultaneously optimized. In preferred embodiments of the invention, the dispersion characteristics are deliberately made selectively dependent on the core radius. This allows manufacturing variability in the dispersion properties, introduced in the core-making process, to be mitigated during subsequent processing steps.

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.10/353762, filed Jan. 29, 2003.

FIELD OF THE INVENTION

This invention relates to optical fibers having improved opticaltransmission characteristics, methods for their production, andcommunication systems incorporating the improved optical fibers.

BACKGROUND OF THE INVENTION

Optical transmission systems employ Wavelength Division Multiplexing(WDM) to increase information handling of an optical fiber transmissionline, typically a long haul transmission line. Early WDM systemsoperated with a relatively narrow wavelength bandwidth, centered around1550 nanometers, e.g. 1530-1565 nanometers, referred to as the C-band.This is the wavelength region where standard silica based optical fibershave optimally low absorption.

In most WDM systems there is a trade-off between the number of channelsthe system accommodates and the channel separation. Higher bit ratesgenerally call for an increase in channel spacing. Both goals favor awide operating spectrum, i.e. a wide range of operating wavelengths.

Recently, systems have been designed that extend the effective operatingwavelength range well above the C-band transmission band. In terms ofwavelength, the new band, referred to as the L-band, is variouslydefined, but for the purpose of this description is 1570-1610nanometers. Substantial work has also been done in the S-band, definedas 1460-1530 nm. Use of these added wavelengths substantially extendsthe capacity of WDM systems. There is an ongoing effort to furtherextend the effective operating wavelength window to above 1610 nm, forexample to 1620 nm. Success of these efforts will depend on findingcomponents, for example amplifiers, that provide effective operationover this broad wavelength range. It is now well appreciated that atransmission fiber should have a minimum level of dispersion at signalwavelengths to enable WDM transmission by suppressing four wave mixingimpairments. Since the dispersion of a non-zero dispersion-shifted NZDFtypically increases toward longer wavelength, this requirement impliesthat the zero dispersion wavelength should be 20-40 nanometers lowerthan the shortest wavelength intended for WDM.

In WDM systems, it is important to have uniform gain over the entire WDMwavelength band. This objective becomes more difficult to reach as theoperating wavelength range is extended to longer and/or shorterwavelengths. Recently, new types of optical fiber amplifiers have beendeveloped that operate using stimulated Raman scattering. The mostprominent of these is a distributed amplifier that operates over thenormal transmission span as a traveling wave amplifier. Raman scatteringis a process by which light incident on a medium is converted to lightat a lower frequency (Stokes case) than the incident light. The pumpphotons excite the molecular vibrations of the medium up to a virtuallevel (non-resonant state). The molecular state quickly decays to alower energy level emitting a signal photon in the process. Because thepump photon is excited to a virtual level Raman gain can occur for apump source at any wavelength. The difference in energy between the pumpand signal photons is dissipated by the molecular vibrations of the hostmaterial. These vibrational levels determine the frequency shift andshape of the Raman gain curve. The frequency (or wavelength) differencebetween the pump and the signal photon is called the Stokes shift. InGe-doped silica fibers, the Stokes shift at which the maximum gain isobtained is ˜13 THz. Due to the amorphous nature of silica the Ramangain curve is fairly broad in optical fibers.

Since Raman scattering can occur at any wavelength; this can beexploited to advantage in a telecommunication system that containsmultiple signal wavelengths by using Raman pumps at several differentwavelengths to amplify the signals. The gain seen by a given wavelengthis the superposition of the gain provided by all the pumps, taking intoaccount the transfer of energy between the pumps due to Ramanscattering. By properly weighting the power provided at each of theRaman pump wavelengths it is possible to obtain a signal gain versuswavelength profile in which there is a small difference between the gainseen by different signal wavelengths (this difference is called the gainripple or gain flatness). The use of Raman amplification thus enablesdense WDM (DWDM) outside the erbium window. Raman amplification is alsoan enabling technology for the evolution from 10 to 40 Gb/s transmissionbecause it improves optical signal to noise ratio at lower launchpowers.

A multiplicity of pumps has been used successfully in many systems.However there is one persistent problem with multiple pumps. An unwantednonlinear effect called four-wave mixing (FWM) may sometimes occur. Intelecommunications systems, if FWM occurs in the signal band this canlead to transmission errors. As the number of pumps in a multi-pumpwavelength Raman amplification scheme increases, the likelihood of FWMincreases.

The harmful effects of four-wave mixing have been recognized. Recentlyone approach towards reducing these effects has been proposed [EP 1 148666 A2]. In this approach the pump wavelengths are either time divisionmultiplexed (TDM) together, or the frequency of the pump source ismodulated (FM). Since the various pump wavelengths overlap for only asmall distances along the fiber, FWM between the pump wavelengths shouldbe eliminated or severely reduced.

While this approach would eliminate FWM, the nominal pump powerrequirements in this system are relatively high. Moreover, to TDM arelatively large number of pump wavelengths, some operating atrelatively high power, adds significantly to the cost and complexity ofthe system. Other approaches to reducing FWM and other non-lineareffects would significantly advance the art.

At least equally as important as compatibility with amplifier technologyin the design of optical fibers for high bit rate, wide-band, systems ismanagement of chromatic dispersion. This problem grows significantly asthe data bit rate is increased. An optical transmission line, comprisinga cabled fiber and a dispersion compensation element (typically a modulebut possibly a cabled fiber), that transmits effectively at 10 Gb/s mayshow excessive error rates at 40 Gb/s because of bit overlap. Fornon-return-to-zero modulation, a 10 Gb/s system should accumulate lessthan ˜1000 ps/nm chromatic dispersion over the total link distance; fora 40 Gb/s system this requirement is tightened to less than 60 ps/nm.

This requirement is met by a combination of two methods. First, in NZDFfibers, dispersion is reduced in the C-band below that of standardmatched clad fiber. To gain this benefit over multiple bands, it isadvantageous that the slope of the dispersion be low. Second, dispersioncompensation technology is employed, most commonly in the form of adispersion compensating fiber (DCF) in a module. For broadbandoperation, it is important that the dispersion curve of the DCF “match”that of the transmission fiber in the appropriate sense. In general,precise compensation of chromatic dispersion over a broad band isachieved when the ratio of the dispersion slope to the dispersion atband center is equal for the fiber and DCF. Furthermore, the bestresults are obtained when this ratio is low. This further emphasizes theadvantage of reduced dispersion slope.

A problem arises in designing optical fibers to meet this general need:typical optical fiber profiles that are optimized for low dispersionslope have reduced effective area due to bend loss constraints. Opticalfibers with reduced effective area generally show increased and unwantednon-linear effects including four-wave mixing as well as self- andcross-phase modulation (SPM, XPM). For Raman amplified systems, toosmall effective area exacerbates the issues of “Raman gain tilt” wherebyshorter wavelength pumps (signals) transfer energy to longer wavelengthpumps (signals).

Thus the manufacture of optical fibers for high bit rate (e.g. 40 Gb/s)systems and with both low dispersion slope and medium or large effectivearea, while at the same time preserving other performancecharacteristics such as low Polarization Mode Dispersion (PMD), is adesign challenge.

STATEMENT OF THE INVENTION

Trade-offs in the parameters just noted to yield improved opticaltransmission performance have been achieved. The improved optical fiberof the invention exhibits these characteristics:

-   -   Dispersion at 1550 nm: 5-8.5, pref. 6.5-7.8, and pref. 7.3±2.0,        ps/nm-km    -   Dispersion slope at 1550 nm: <0.045 ps/n m²-km, pref. <0.042        ps/nm²-km    -   Effective area at 1550 nm: >50, pref. 54-62, and pref. 54-58 μm²    -   Cable cutoff wavelength: <1410 nm    -   Microbend loss (32 mm) at 1625 nm: <5×10³ dB/km    -   Zero dispersion wavelength: <1400 nm

This set of properties in general represents optical fibers withmoderate chromatic dispersion and moderate effective area to minimizenon-linear effects, and low dispersion slope for ease of precise,wideband, dispersion compensation. It also represents fibers designedfor distributed Raman amplification and/or S-, C-, and L-band operationwhile at the same time being compatible with erbium-doped fiberamplifiers (EDFAs).

A variety of optical fiber refractive index profiles that produce thesetransmission characteristics have been designed. In general these have acomplex core comprising an up-doped central core (usually referred to asthe core), surrounded by a down-doped region (usually referred to as thetrench), further surrounded by an up-doped region (referred to as thering). A similar basic profile (but with different performancecharacteristics) is described and claimed in U.S. Pat. No. 5,878,182,and U.S. Pat. No. 5,905,838, which are incorporated herein by reference.In advanced fiber designs, the profile may also have a second down-dopedtrench, width a width of, for example, 2-10 microns, added either justoutside the ring, or farther out in the cladding, in order to adjust thecutoff wavelength, and reduce microbending loss.

In the preferred manufacturing method, the size of the core region isadjusted during manufacture to achieve the design objectives. Theoptical parameters of the fiber are specifically designed so that thedispersion characteristics of the fiber are selectively dependent on thecore radius. This allows the dispersion characteristics to beselectively adjusted by changing the core radius during manufacture. Thepreferred manufacturing methods employ rod-in-tube techniques for makingthe optical fiber preform. After forming the core rod, it rod may besubjected to a plasma treatment, or other suitable process step, tomodify the core diameter.

In a preferred embodiment of the invention the core rod is made using anMCVD method and the core consequently exhibits a so-called alphaprofile, with an index that varies with core radius, and a maxima in thecenter half of the core (the first half of the core radius measured fromthe core center). However, optical fibers meeting the performanceparameters of the invention, may have any suitable refractive indexprofile, and may be manufactured by any of a variety of methods, such asfor example, OVD, VAD, PCVD, POVD, MCVD. Combinations of the abovemethods may be used for fabricating the various regions of the indexprofile and overclad regions as appropriate, including the incorporationof soot-on-glass, soot-on-soot, or glass-on-glass interfaces in thepreform as appropriate.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified diagram of an optical communications system witha Raman optical fiber amplifier and an optical fiber designed accordingto the invention;

FIG. 2 shows an alternative multiple pump arrangement for the Ramanamplifier of FIG. 1;

FIGS. 3-8 are optical fiber profile designs according to the invention;

FIGS. 9 and 10 are schematic representations of a rod-in-tube processfor the manufacture of optical fiber preforms;

FIG. 11 is a schematic representation of the step of adjusting the corediameter according to the invention; and

FIG. 12 is a schematic representation of a fiber drawing apparatususeful for drawing preforms made by the invention into continuouslengths of optical fiber.

DETAILED DESCRIPTION

Referring to FIG. 1, an optical fiber communications system is shownwith a distributed Raman optical fiber amplifier. The transmission span11 represents a fiber of substantial length, typically in excess of 1km. It will be evident to those skilled that the figures in thisdescription are not drawn to scale, and the elements are schematicallyshown. For purposes of illustration, FIG. 1 shows a distributedamplifier where the amplifier medium is the normal transmission span.For discrete amplifiers, a dedicated length of optical fiber can be usedfor the amplification medium. The length of fiber represented by 11 istypically at least 500 m in length to allow for the optical interactionsthat produce signal amplification. The amplifier is end pumped, andcounter pumped, as shown in the figure, by pump source 13 coupled intothe core of the fiber through coupler shown schematically at 12. Thesystem transmitter is shown at 14 and the system receiver at 16. Adispersion compensating module may be included at 15.

Optical fiber Raman amplifiers operate on the principle that lightscattered in a silica based optical fiber has a wavelength lower thanthat of the incident light. Pump photons excite molecules up to avirtual level (non-resonant state). The excited molecules quickly decayto a lower energy level (Stoke's case) emitting signal photons in theprocess. Because the pump photon is excited to a virtual level, Ramangain can occur for a pump source at any wavelength. The difference inenergy between the pump and signal photons is dissipated by themolecular vibrations of the host material. These vibrational levelsdetermine the frequency shift and shape of the Raman gain curve. Thefrequency (or wavelength) difference between the pump and the signalphoton is called the Stokes shift. In Ge-doped silica fibers, the Stokesshift at which maximum gain is obtained is ˜13 THz. Due to the amorphousnature of silica the Raman gain curve is fairly broad in optical fibers.

In a telecommunication system that contains multiple signal wavelengthsRaman pumps at several different wavelengths may be used to amplify thesignals, since Raman scattering can occur at any wavelength. The gainseen by a given wavelength is the superposition of the gain provided byall the pumps taking into account the transfer of energy between thepumps due to Raman scattering. By properly weighting the power providedat each of the Raman pump wavelengths it is possible to obtain a signalgain versus wavelength profile in which there is a small differencebetween the gain seen by different signal wavelengths. This differenceis called the gain ripple or gain flatness, and may be expressed in dBas (Gmax−Gmin).

FIG. 1 shows a Raman amplifier system using a single, counter pump. Thesystem may also be co-pumped, with pumps from either direction. Theadvantages of this approach was recently pointed out in (U.S. Pat. No.6,163,630). Multiple co- and counter pumps may be multiplexed to improvegain flatness as previously described. In multiple order Raman pumpingthe signal light is greater than 1.5 Stokes shift away from the maximumgain frequency of the pump light. As an example in 2^(nd) order pumping,a pump wavelength 2 Stokes shifts away from the signal light is used topump a 1^(st) order Stokes pump that is 1 Stokes shift away from thesignal light. This is illustrated in FIG. 2, where both a 1^(st) and2^(nd) order pump are counter pumped relative to the signal light. Ittakes a finite length of fiber for the 2^(nd) order pump to be convertedto the 1st order pump. The 1^(st) order pump then pumps the signal. Thisthen allows the signal amplification to occur closer to the signal inputend of the fiber. Multiple order pumping is advantageous because infirst order Raman pumping the pump generally travels in the oppositedirection of the signal. Most of the amplification occurs near thesignal output end of the transmission span. At this position in thefiber the signal power has already significantly degraded. If the Ramangain seen in the fiber can occur closer to the signal input end of thefiber an improved signal to noise ratio (SNR) and noise figure (NF) isobtained. The power needed for a second order pump is fairly modest. Inone example of a dual order pumped system, the power ratio for a1366/1455 nm pump was 970/10 mW respectively.

The use of multiple pumps, however, introduces the problem of four wavemixing (FWM). Four-wave mixing occurs when photons of two or more wavescombine to create photons at other frequencies. The new frequencies aredetermined as such that total energy and momentum (phase matching) isconserved. FWM may result from non-linear interaction between two ormore pump wavelengths.

In a telecommunications system, spurious wavelength components resultingfrom FWM in the signal band may lead to transmission errors. UnlikeRaman scattering in which the phase matching conditions areautomatically satisfied, the efficiency of FWM depends on a properchoice of frequencies and refractive indices. There are threecontributions to the phase mismatch; material dispersion, waveguidedispersion and fiber nonlinearity. By adjusting the location of the zerodispersion wavelength (hence the waveguide dispersion) of the fiber, FWMcan be controlled, and in many cases, practically eliminated. Ingeneral, it is desirable to have the zero of dispersion at a wavelengthshorter than the shortest wavelength pump, so that the dispersion isgreater than or equal to 1 or 2 ps/nm/km over the entire region of Ramanpumps and signals.

An effective approach towards reducing FWM, SPM, XPM, and interbandstimulated Raman effects is to minimize the non-linear properties of theoptical fiber itself. This may be achieved by increasing the effectivearea, A_(eff), of the optical fiber. In doing this, a variety oftrade-offs should be considered, including bend losses and cutoff. Ingeneral, there is a limit beyond which increasing the effective area,while maintaining acceptable bend losses and cutoff, will sacrificelower dispersion slope and its attendant benefits previously discussed.

It has been noted that the particular dispersion vs. wavelength curve ofan optical fiber design determines how precisely chromatic dispersioncan be compensated over a wideband, especially in the case ofsingle-mode dispersion compensating fiber solutions. A relevant anduseful parameter is the ratio of the dispersion slope to the dispersionat the central wavelength of the signal band (here called the “relativedispersion slope” or RDS). If the RDS of the cabled transmission fiber(typically with positive dispersion) is equal to the RDS of the negativedispersion compensating fiber (typically housed in a module, but can becabled as well), then precise cancellation of dispersion can be achievedover a wide range (e.g. ±15 to 20 nm). In general the best results canbe achieved when the transmission fiber has a relatively low value ofRDS. First, it is difficult to fabricate dispersion compensating fiberswith high RDS. Second, the dispersion vs. wavelength relation for highRDS fibers are typically more curved than for lower RDS fibers, leadingto significant error in compensation for 40 Gb/s transmission.

SSMF fiber has RDS ˜0.0033/nm at 1550 nm, defining the lower end of thescale. Commercial NZDF fibers with Aeff>50 sq microns have RDS rangingfrom 0.0065/nm to 0.02/nm at 1550 nm, with full C-band compensationproving very difficult to realize for the high end values of RDS. Thepresent invention advances the state of the art by reducing RDS acrossthe S-, C-, and L-bands, supporting innovation in wideband dispersioncompensation solutions. In addition to enabling more precise wide bandcompensation across the C and L bands, the very low RDS ˜0.0050/nm at1570 makes possible combined C+L band module, while the low RDS˜0.0075/nm at 1510 nm enables compensation in the upper S-band. Adesirable prescription for RDS over the bands of interest is:

-   -   0.0064 to 0.0082 per nm at a wavelength of approximately 1510 nm        (S-band application)    -   0.0046 to 0.0058 per nm at a wavelength of approximately 1550 nm        (C-band application)    -   00.42 to 00.54 per nm at a wavelength of approximately 1570 nm        (C+L band compensation with combined module)    -   0.0038 to 0.0048 per nm at a wavelength of approximately 1590 nm        (L-band application)

It is important not only to compensate the nominal chromatic dispersionof the fiber, but also to minimize the usual manufacturing variabilitythat leads to dispersion non-uniformity along the length of the fiberand between pieces of the same fiber type. Typical commercial productionspecifications on dispersion at 1550 nm range from ±0.75 to 1.25ps/nm/km. Dispersion varies because the final index profile in the fibervaries from the ideal targets, such as those shown in FIGS. 3-8. Theimpact over many hundreds of kilometers in a transmission system is anaccumulated error in dispersion compensation, leading to greater costexpended to trim the dispersion values to specification at the terminalsof the system.

It is thus of great advantage to minimize manufacturing variability indispersion. Most fiber refractive index profile non-idealities originatein the core rod fabrication. However, there are practical techniques tocorrect certain types of errors prior to the draw process. The presentinvention facilitates this goal by designing the refractive indexprofile to have a pattern of sensitivities to core index errors that issuitable to correction by these techniques.

The index profile parameter most susceptible to control by thesetechniques is the core radius. It is generally impractical or impossibleto adjust the refractive index of the core rod after fabrication.However, it is possible to adjust the core diameter at an intermediatestage in the core rod fabrication step sequence prior to overcladding.The latter mechanism is described and claimed in U.S. patent applicationSer. No. 09/567,536, filed May 9, 2000, which is incorporated herein byreference.

Thus it is beneficial to determine a set of parameters in which thetransmission property of dispersion, the ultimate parameter to becontrolled, will vary strongly with core radius, but only weakly withother design parameters.

Generally speaking, assume that Y is a variable denoting an opticalproperty of the fiber, and X is a variable denoting a refractive indexprofile parameter. The derivative of Y with respect to X, denoted ∂Y/∂X,indicates the sensitivity of fiber property Y to the index parameter X.The objective is for dispersion to vary more strongly with core radiusthan with any other index profile parameter, and for no other opticalproperty to be strongly dependent on core radius. Thus:∂D/∂w₁>4×∂Y/∂X, where X≠outer radius of core regionwhere D denotes dispersion and w₁ denotes outer radius of the fiber coreregion. With this condition in place, the core radius can be adjusted asmentioned to effectively control the dispersion characteristics of theoptical fiber without introducing variation along the fiber into othertarget optical properties, such as dispersion slope or mode field. Atthe same time it is desired that other properties, such as refractiveindex in the core, have substantially less effect. The condition ofrelatively high dependence of dispersion D on core radius a₁ may beexpressed, as just stated, as the ratio ∂D/∂r, where, with D in ps/nm-kmand r in microns, the ratio desired is at least 5 and preferably 10 ormore.

Examples of index profiles meeting the requirements of the invention areshown in FIGS. 3-7. The profiles are shown as preform design profiles(the preform OD is typically 63 mm). However, optical fibers producedfrom these preforms essentially replicate these profiles, but withsmaller dimensions. In all cases, the properties of optical fibersproduced using these preforms fall within the following prescription:

-   -   Dispersion at 1550 nm: 7.3±1.0 ps/nm-km    -   Dispersion slope at 1550 nm: <0.042 ps/nm²-km, 0.041 typical    -   Effective area at 1550 nm: 54-62 μm²    -   Cable cutoff wavelength: <1410 nm    -   Microbend loss (32 mm) at 1625 nm: <5×10³ dB/km    -   Zero dispersion wavelength: <1400 nm

The optical fiber profiles basically comprise four regions. These areshown in FIG. 3, for example, as core region 21, trench region 22, ringregion 23 and cladding 24.

Core Region

The core consists of a raised index region extending from the centralaxis of the preform to radius a, with the radial variation of thenormalized index difference, Δr, described by the equation:Δr=Δ(1−(r/a)^(α))−Δ_(dip)((b−r)/b)^(y)   (1)where

-   -   r is the radial position,    -   A is the normalized index difference on axis if Δ_(dip)=0,    -   a is the core radius,    -   α is the shape parameter,    -   Δ_(dip) is the central dip depth,

The parameters Δ_(dip), b, and γ, i.e. the central dip depth, b thecentral dip width, and the central dip shape are artifacts of MCVDproduction methods, and these factors may be used when MCVD methods arethe production choice for the optical fiber preform. When using otherpreform fabrication techniques, for example VAD, there will be nocentral dip.

The equation describing the core shape consists of the sum of two terms.The first term generally dominates the overall shape and describes ashape commonly referred to as an alpha profile. The second termdescribes the shape of a centrally located index depression (depressedrelative to the alpha profile). The core region generally consists ofsilica doped with germanium at concentrations less than 10 wt % at theposition of maximum index, and graded with radius to provide the shapedescribed by equation (1).

Nominal values for the above parameters that yield fiber with thedesired transmission properties are:Δ=0.50%, a=3.51 μm, α=12, Δ_(dip)=0.35%, b=1.0 μm, y=3.0

In general, the range of variation for these parameters may be:Δ=0.30˜0.70%a=2.0˜4.5 μmα=1˜15The Trench Region

The trench region is an annular region surrounding the core region withan index of refraction that is less than that of the SiO₂ cladding. Theindex of refraction in this region is typically approximately constantas a function of radius, but is not required to be flat. The trenchregion generally consists of SiO₂, doped with appropriate amounts offluorine and germania to achieve the desired index of refraction andglass defect levels.

The nominal trench parameters are:Δ=−0.21% and width=2.51 μm.

In general, the range of variation for these parameters may be:Δ=−0.25˜−0.10%a=4.0˜8.0 μmThe Ring Region

The ring region is an annular region surrounding the trench region withan index of refraction that is greater than that of the SiO₂ cladding.The index of refraction in this region is typically constant as afunction of radius, but is not required to be flat. The ring regiongenerally consists of SiO₂, doped with appropriate amounts of germaniato achieve the desired index of refraction.

The nominal ring parameters are:Δ=0.18% and width=2.0 μm

In general, the range of variation for these parameters may be:Δ=−0.10˜−0.60%a=7.0˜10.0 μmThe Cladding Region

The cladding region is an annular region surrounding the ring, usuallyconsisting of undoped SiO₂. However, internal to the cladding region mayalso exist an additional region of fluorine doped glass, of theappropriate index level and radial dimensions, to improve bending losscharacteristics. The cladding region generally extends to 62.5 μmradius.

An idealized preform profile meeting the requirements of the inventionis shown in FIG. 8. Here the core region is shown at 31, the trenchregion at 32, the ring region at 33, and the undoped cladding at 34. Thecharacteristic center dip, not an ideal feature, is represented by thedashed lines 35.

The variations of the major transmission properties over the variationof the index profile parameters for this design are as follows: (f = D)(f = Aeff) (f = DS) df (df/f × 100) df (df/f/ × 100) df (df/f/ × 100)df/dN1   0.40(5.6) −1.237(−2.2)   0.000(−0.7) df/dW1   1.56(22.2)−0.017(0.0)   0.001(1.6) df/dN2   0.16(2.3)   0.497(0.9)   0.001(3.0)df/dW2   0.05(0.7) −0.580(−1.0) −0.001(−3.6) df/dN3 −0.17(−2.4)  0.380(0.7)   0.000(0.9) df/dW3 −0.19(−2.7)   0.301(0.5)   0.000(0.1)dD/dW = change in D resulting from a 0.1 micron change in width (W)dD/dN = change in D resulting from a 0.0001 change in relative delta (N)D = dispersion at 1550 nmAeff = effective area at 1550 nmDS = dispersion slope at 1550 nm

It is evident that the core radius is the dominant parameter thataffects the transmission property of dispersion, while variations inother profile parameters do not have as much effect. This means that anintelligent core diameter adjustment to the fabricated rod may beapplied, after it is measured, to correct errors in the profile. Suchadjustments may result in improved manufacturing yields, lower costs,and better system performance. The choice of 0.1 micron width variationand 0.0001 as the scale for index variation in this example places thederivatives with respect to these two different parameters on equalfooting for comparison, since these levels of variation correspond tothe typical standard deviations for real manufactured fibers.

It is also evident that the manufacturing expedient just described, i.e.adjusting core diameter during preform manufacture, is applicable to arod-in-tube preform manufacturing process. Those skilled in the art maydevelop techniques for adjusting core diameter in other manufacturingtechniques, but making the adjustment in a rod-in-tube process is thepreferred case. This approach also allows the core and inner claddingregions to be formed using MCVD, a preferred choice from the standpointof quality and performance of the finished fiber.

Typical rod-in-tube methods are described in conjunction with FIGS. 9and 10. It should be understood that the figures referred to are notnecessarily drawn to scale. A cladding tube representative of dimensionsactually used commercially has a typical length to diameter ratio of10-15. The core rod 42 is shown being inserted into the cladding tube41. The tube 41 may represent a single tube or several concentric tubes.The rod at this point is typically already consolidated. The tube may bealready consolidated or still porous. Normally, there exist severalcommon options for the make-up of the core rod. It may be just thecenter core, or it may include one or more of the layers 32, 33 in FIG.8. In the embodiment of the invention where the rod consists of just thecore, 31 in FIG. 8, the remaining doped layers may be formed by one ormore cladding tubes. Cladding tubes made with very high qualityglass-forming techniques may be used for trench and ring layers, as wellas the cladding layers.

After assembly of the rod 42 and tube 41, the combination is sintered toproduce the final preform 43, shown in FIG. 10, with the core rod 44indistinguishable from the tube or tubes except for a small refractiveindex difference. This may occur either prior to or during the drawprocess.

Since the rod 42 either has only the core region, or contains the coreregion, the diameter of the core region may be adjusted before insertionin the tube. The adjustment is determined by measuring of the opticalcharacteristics of the core rod to find the actual deposited core radiuscharacterizing region 31 in FIG. 8, and calculating the differencebetween the actual and desired core radius. The desired core radius maybe taken from one of the profiles shown in FIGS. 3-7.

The core adjustment step is represented by FIG. 11, where the rod 42,with initial diameter D₁, is processed to change the rod diameter andproduce a rod with diameter D₂. Diameter D₂ is the desired core rodouter diameter, which produces the correct diameter of deposited core(region 31 in FIG. 8). The diameter change may be produced by anysuitable method. The outer core material may be removed by machining orby plasma etching. In a preferred embodiment, the diameter of the coreregion of the rod is modified by traversing the rod with a plasma torchwith the rod under longitudinal stress. This step is procedurallysimilar to that used in standard MCVD for tube collapse, and is wellknown in the art. The stress may be compressive or tensile, depending onthe result of the measurement just described. This produces a strain ΔLin the rod length L. Since the amount of material in the rod is fixed,any strain ΔL will produce a change in the core diameter. In the caseillustrated in FIG. 11, the core radius is reduced from D₁ to D₂ bystress S applied in a tensile mode.

For optimum preform manufacture, essentially all of the preforms will beprocessed to “trim” the core diameter to the values of thespecification. The method involves preparing a “conditional” rod, withdiameter D₁, measuring the conditional rod to determine the rod diameteradjustment value (D₁−D₂), making the adjustment, and completing therod-in-tube method. In some cases variations in optical properties alongthe length of the rod will be found. These may be corrected by changingthe stress on the rod as the plasma traverses the length of the rod, orby changing the plasma torch conditions, or traverse speed, duringtraverse.

When the method used for core diameter adjustment involves removal ofmaterial from the outer surface of the rod (for example, the shellregion represented by D₁−D₂ in FIG. 9) and it is anticipated that allrods will require adjustment, the diameter D₁ is preferably madedeliberately larger than the specification, to eliminate the possibilityof producing undersized rods. If the rod is undersized, the rod diametercannot be corrected by material removal. However, even in that event,preform rods can be salvaged by depositing additional core material onthe undersized rod.

It should be mentioned that plasma treatment of core rods is desirable,in addition to being the method of choice for diameter adjustment bystretching/compression, because at least some material will be removedfrom the outer surface of the core rod. This surface typically iscontaminated with OH⁻ and removing the contaminant by plasma treatmentis desirable regardless of other processing choices.

The optical fiber preform, as described above, is then used for drawingoptical fiber. FIG. 12 shows an optical fiber drawing apparatus withpreform 81, and susceptor 82 representing the furnace (not shown) usedto soften the glass preform and initiate fiber draw. The drawn fiber isshown at 83. The nascent fiber surface is then passed through a coatingcup, indicated generally at 84, which has chamber 85 containing acoating prepolymer 86. The liquid coated fiber from the coating chamberexits through die 91. The combination of die 91 and the fluid dynamicsof the prepolymer controls the coating thickness. The prepolymer coatedfiber 94 is then exposed to UV lamps 95 to cure the prepolymer andcomplete the coating process. Other curing radiation may be used whereappropriate. The fiber, with the coating cured, is then taken up bytake-up reel 97. The take-up reel controls the draw speed of the fiber.Draw speeds in the range typically of 1-30 m/sec. can be used. It isimportant that the fiber be centered within the coating cup, andparticularly within the exit die 91, to maintain concentricity of thefiber and coating. A commercial apparatus typically has pulleys thatcontrol the alignment of the fiber. Hydrodynamic pressure in the dieitself aids in centering the fiber. A stepper motor, controlled by amicro-step indexer (not shown), controls the take-up reel.

Coating materials for optical fibers are typically urethanes, acrylates,or urethane-acrylates, with a UV photoinitiator added. The apparatus ofFIG. 12 is shown with a single coating cup, but dual coating apparatuswith dual coating cups are commonly used. In dual coated fibers, typicalprimary or inner coating materials are soft, low modulus materials suchas silicone, hot melt wax, or any of a number of polymer materialshaving a relatively low modulus. The usual materials for the second orouter coating are high modulus polymers, typically urethanes oracrylics. In commercial practice both materials may be low and highmodulus acrylates. The coating thickness typically ranges from 150-300μm in diameter, with approximately 245 μm standard.

It should be emphasized that, while the invention has been describedlargely in the context of MCVD and rod-in-tube processing, the actualmethod used to achieve the results that form a basis for one aspect ofthe invention may be selected from a wide variety of choices. Theseinclude, but are not limited to, the use of OVD, VAD, PCVD, POVD, MCVDand combinations thereof; the use of different refractive index profilesto achieve the end properties claimed, and other similar alternatives.These and other additional modifications of this invention will occur tothose skilled in the art. All deviations from the specific teachings ofthis specification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

1. An optical WDM system using Raman amplification comprising: a. alength of optical fiber, b. lightwave signal means for introducing alightwave signal into the optical fiber, the lightwave signal comprisingat least three wavelength division multiplexed (WDM) wavelengths, c.optical pump means for introducing lightwave pump energy into the coreof the glass fiber, whereby the lightwave pump energy interacts with thelightwave signal to produce Raman amplification of the lightwave signal,the invention characterized in that the optical fiber comprisessequential regions of: i. a core region extending from the center of theoptical fiber, with essentially all of the core region having a positiveΔ, ii. a trench region, with essentially all of the trench region havinga negative Δ, iii. a ring region having a positive Δ, and the opticalfiber is characterized by: Dispersion at 1550 nm: 5-8.5 ps/nm-kmDispersion slope at 1550 nm: <0.045 ps/nm²-km Effective area at 1550nm: >50 μm² Cable cutoff wavelength: <1410 nm Macrobend loss (32 mm) at1625 nm: <5×10³ dB/km Zero dispersion wavelength: <1400 nm.
 2. Thesystem of claim 1 wherein the signal means includes wavelengths above1510 nm, and includes 1550 nm.
 3. The system of claim 2 wherein:Dispersion at 1550 nm=7.3±2 ps/nm-km
 4. The system of claim 2 wherein:Effective area at 1550 nm=54-62 μm²
 5. The system of claim 2 wherein:Dispersion slope at 1550 nm: <0.042 ps/nm²-km
 6. The system of claim 2wherein: Dispersion at 1550 nm=7.3±2 ps/nm-km Effective area at 1550nm=54-58 μm² Dispersion slope at 1550 nm: <0.042 ps/nm²-km
 7. The systemof claim 1 wherein the system includes at least one erbium-doped fiberamplifier.
 8. The system of claim 1 wherein the at least threewavelength division multiplexed (WDM) wavelengths operate over the S-,L- or extended L-bands.
 9. The system of claim 1 wherein the lightwavesignal means operates at 40 Gb/s.
 10. The system of claim 1 furtherincluding means for dispersion slope compensation.
 11. An optical WDMsystem comprising: a. a length of optical fiber, b. lightwave signalmeans for introducing a lightwave signal into the optical fiber, thelightwave signal comprising at least three wavelength divisionmultiplexed (WDM) wavelengths, c. means for dispersion slopecompensation having a relative dispersion slope, defined as dispersionslope divided by dispersion at a given wavelength matched to the opticalfiber, of: 0.0064 to 0.0082 per nm at a wavelength of approximately 1510nm (S-band application) 0.0046 to 0.0058 per nm at a wavelength ofapproximately 1550 nm (C-band application) 00.42 to 00.54 per nm at awavelength of approximately 1570 nm (C+L band compensation with combinedmodule) 0.0038 to 0.0048 per nm at a wavelength of approximately 1590 nm(L-band application) the invention characterized in that the opticalfiber comprises sequential regions of: i. a core region extending fromthe center of the optical fiber, with essentially all of the core regionhaving a positive Δ, ii. a trench region, with essentially all of thetrench region having a negative Δ, iii. a ring region having a positiveΔ, and the optical fiber is characterized by: Dispersion at 1550 nm:5-8.5 ps/nm-km Dispersion slope at 1550 nm: <0.045 ps/nm²-km Effectivearea at 1550 nm: >50 μm² Cable cutoff wavelength: <1410 nm Macrobendloss (32 mm) at 1625 nm: <5×10³ dB/km Zero dispersion wavelength: <1400nm.
 12. The system of claim 11 wherein the C-band and L-bandcompensation is achieved with a single means having an RDS of 00.42 to00.54 per nm at a wavelength of approximately 1570 nm.